Organopolysiloxanes, modified with acrylate ester groups (acrylate groups), have proven their value as high energy radiation-curable binders, for example for printing inks and for the manufacture of lacquer vehicles or coating agents for paper, wood and metal surfaces. In particular, these can be employed as abhesive coating agents. Curing takes place in a very short time particularly after the addition of known initiators, such as benzophenone and its derivatives, by UV radiation or by electron beam curing.
The acrylate-containing groups can be bonded to the backbone of the organopolysiloxane by way of SiOC or SiC bonds. In this connection, there is an extensive state of the art, of which the following patents have been selected as typical examples.
Organopolysiloxanes, the acrylate ester-containing organic groups of which are linked to the polysiloxane backbone over SiOC bonds, can be synthesized according to a method of the German patent 27 47 233 by reacting (meth)acrylate esters, having --COH groups, with organopolysiloxanes, having SiX groups, wherein X is an alkoxy, hydroxy or chlorine group, owing to the fact that organopolysiloxanes of the formula ##STR3## wherein R.sup.1 is an alkyl group with 1 to 4 carbon atoms and/or a phenyl group;
X is chlorine or OR.sup.2 group; PA1 R.sup.2 is an alkyl group with 1 to 4 carbon atoms and/or hydrogen; PA1 a is 1.0 to 2.0; PA1 b is 0.02 to 1.6; and PA1 a+b is at least 2.66; PA1 R.sup.5 represents linear or branched alkylene groups with 1 to 8 carbon atoms, it being possible for these groups to be adjacent to one another in the average molecule, PA1 R.sup.2 is a (c+1) valent hydrocarbon bridge with 1 to 20 carbon atoms, which may have ether groups and to which lateral methacrylate groups may be linked, PA1 Q is a diol or polyol group with 1 hydroxyl group less, the remaining hydroxyl group(s) of which is (are) esterified with (meth)acrylic acid the diols or polyols being selected from the group comprising ethylene glycol, propylene glycol, glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and their dimers and polymers formed by intermolecular condensation, ethylene oxide and/or propylene oxide optionally being added in an addition reaction to these polyols, PA1 a is 1.6 to 2.2, PA1 b is 0.001 to 1.6, PA1 c is 1 to 4, and PA1 a+b is at most 3. PA1 a) organopolysiloxanes of the general formula ##STR7## wherein R.sup.1, R.sup.2, a, b and c are as already defined, with, based on the hydroxyl groups, at least equimolar amounts of compounds of the general formula ##STR8## wherein R.sup.3 is a chlorine group or an alkoxy group with 1 to 4 carbon atoms, optionally removing the excess amounts of the compound ##STR9## and reacting the reaction product thus obtained with, based on the R.sup.3 group, equimolar amounts of the compound of the general formula Q--OH, in which Q is as already defined, or PA1 b) initially reacting compounds of the formula Q--OH with at least equimolar amounts of the compound of the formula ##STR10## and reacting the reaction product with equimolar amounts of organopolysiloxane under reaction conditions otherwise described in a). PA1 X is a halogen group, preferably the chlorine group, PA1 R.sup.3 is an alkyl group with 1 to 20 carbon atoms, an alkenyl group with 2 to 20 carbon atoms or the R.sup.3.sup.3 SiQ group, in which Q is a divalent aliphatic group with 1 to 6 carbon atoms, PA1 x is a number from 0 to 4. PA1 R.sup.2 groups are identical with R.sup.1 groups or are hydrogen groups, with the proviso that at least one R.sup.2 group is a hydrogen group, PA1 and subscript PA1 a has an average value of 1 to 100, and PA1 b has an average value of 1 to 5. PA1 1. The inventive method is a simple one with easily accessible starting materials and a high selectivity for SiC-linked addition products, particularly for .beta. 1,2 products. PA1 2. An excess of acrylate component is not required for the hydrosilylation. PA1 3. The products have outstanding long-term storage stability. PA1 4. High-speed curing of the products on the backing material is possible. PA1 5. According to the release values, there is little or no change in the abhesiveness of the cured products over time.
are used as organopolysiloxane, the siloxane molecule having 3 to 100 silicon atoms and pentaerythritol tri(meth)acrylate being used as (meth)acrylate ester in 0.05 molar to equimolar amounts, based on the COH and SiX groups.
In a modification of this method, the procedure of the German patent 29 48 708 is employed and organopolysiloxanes of the formula ##STR4## (R.sup.1 =alkyl with 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mole percent of the R.sup.1 groups are methyl groups, a has a value of 1.8 to 2.2 and b a value of 0.004 to 0.5) are reacted initially with, based on the SiCl groups, at least 2 moles of a dialkylamine, the alkyl groups of which in each case have 3 to 5 carbon atoms and the carbon atoms of which, adjacent to the nitrogen, carry at most one hydrogen atom each, and the reaction product is then reacted with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol trimethacrylate, after which the product is separated from suspended solid components in it by known methods.
Organopolysiloxanes with acrylate ester-containing organic groups linked over an SiC bond with the polysiloxane backbone, typically, can be synthesized by reacting a hydrogensiloxane in an addition reaction with an alcohol having an olefinic double bond, such as allyl alcohol, in the presence of a platinum catalyst and then reacting the hydroxyl group of this alcohol with acrylic acid or a mixture of acrylic acid and other, optionally saturated acids. This method is described, for example, in the German patent 38 10 140.
A further possibility of synthesizing acrylate-modified polysiloxanes with SiC linkage of the modifying group(s) consists therein that allyl glycidyl ether or a different suitable epoxide with an olefinic double bond is added in an addition reaction to a hydrogensiloxane and, after the addition reaction, the epoxide is esterified with acrylic acid with opening of the epoxide ring. This method is described in the German patent 38 20 294.
The German patent 37 10 238 discloses compounds of the general formula ##STR5## wherein R.sup.1 is an alkyl group with 1 to 4 carbon atoms, a phenyl group or an alkaryl group of the general formula ##STR6## wherein R.sup.4 are the same or different and represent a hydrogen group, an alkyl group with 1 to 12 carbon atoms and/or a halogen group,
These compounds can be synthesized by reacting
Furthermore, the U.S. Pat. No. 4,725,630 discloses the reaction of mercaptopropyl groups with acrylate esters, such as pentaerythritol triacrylate or tetraacrylate. Admittedly, it is possible to add one of the acrylate groups directly to the mercaptopropyl group. However, the method has not attained practical importance, since it is expensive and the products have a very unpleasant odor.
It is well known to those skilled in the art that the acrylate-modified organopolysiloxanes, in which the organic groups, which carry the acrylate group or groups, are linked over SiC bonds to the siloxane backbone, are superior with respect to the resistance to hydrolysis to the compounds in which said organic groups are linked over an SiOC bond.
It would be of considerable advantage for the synthesis of SiC-linked and acrylate-modified organopolysiloxanes, if compounds with at least two acrylate groups could be added directly in an hydrosilylation reaction to hydrogensiloxanes. This procedure cannot be inferred from the patent literature.
In the "Journal of Applied Polymer Science", Volume 47, 1309 to 1314 (1993), a method is described for reacting organic polyacrylates with hydrogensiloxanes. For this method, the organic polyacrylate, namely 1,6-hexylene glycol diacrylate or trimethylolpropane triacrylate is to be used in a considerable excess, so that only one of the two or three olefinic groups reacts with the hydrogensiloxane. This reaction shall be carried out in the presence of platinum catalysts. It was reported in this paper that the excess of polyacrylate is necessary in order to avoid a gelling of the product during the hydrosilylation reaction or the subsequent storage. The product can be cured by radiation, but exhibits a great decrease in the release effect during storage. Such a product is therefore not usable industrially as an abhesive coating composition.
According to the publication, the reaction proceeds as follows: ##STR11##
However, when this method was repeated, it was observed that the reaction proceeded differently from the way described above. Whereas the above reaction equation would be based on a 1,2 addition at the C.dbd.C double bond with formation of an Si--C linkage, spectroscopic investigations showed that, under the given conditions, an addition at the C=O double bond with formation of the 1,2- and 1,4--Si--O--C-linked reaction products takes place. ##STR12##
For the methods described in the aforementioned publications, products with an SiOC link are obtained and not the desired organopolysiloxanes with an SiC link and modified with acrylate groups. It is to be assumed that the .dbd.Si--O--C-linked products are responsible for the gelling and the decrease in release effect upon ageing.
The present invention is concerned with the technical problem of the direct addition of polyacrylates to hydrogensiloxanes by 1,2 addition according to the following reaction equation: ##STR13##
The B addition products (--SiCH.sub.2 --CH.sub.2 --COOR and Si--CH=CH--COOR) are thermodynamically particularly stable and therefore are preferred addition products.
Surprisingly, it was found that the desired Si--C linking takes place when rhodium catalysts are used as catalysts. Moreover, the selectivity for the preferred .beta. 1,2 addition products is very high.